101787
6. Dimensioning metric screw assembliesVDI guideline 2230, published in 2003, provides fundamental information on dimensioning, in particular of high-strength screw assemblies in engineering.
The calculation of a screw assembly starts from the operating force FB that works on the joint from the outside. This operating force and the elastic deformations of the components that it causes bring about an axial operating force FA, a shear force FQ, a bending moment Mb and where applicable a torque MT at the individual screw position.
When the necessary screw dimension is calculated mathematically, it must be taken into account, starting from the known load ratios, that a loss of preload force can occur through setting processes and temperature changes.
It must also be taken into account that, depending on the chosen assembly method and on the frictional conditions, the assembly preload force FM can disperse in more or less wide limits.
An approximate dimensioning is often sufficient for an ini-tial selection of the suitable screw dimension. Depending on the application, further criteria are then to be checked in accordance with VDI 2230.
6.1 Approximate calculation of the dimension or the strength classes of screws (in accordance with VDI 2230)
On the basis of the above-mentioned findings, the pre-selection of the screw is carried out in the first step in accordance with the following table.
1 2 3 4Force in N Nominal diameter in mm
Strength class12.9 10.9 8.8
250 400 630 1.000 M3 M3 M3 1.600 M3 M3 M3
1 2 3 4Force in N Nominal diameter in mm
Strength class12.9 10.9 8.8
2.500 M3 M3 M4 4.000 M4 M4 M5 6.300 M4 M5 M6 10.000 M5 M6 M8 16.000 M6 M8 M10 25.000 M8 M10 M12 40.000 M10 M12 M14 63.000 M12 M14 M16100.000 M16 M18 M20160.000 M20 M22 M24250.000 M24 M27 M30400.000 M30 M33 M36630.000 M36 M39
Tab. 1
A From column 1 choose the next higher force to the one that acts on the joint. If the combined load (lengthwise and shear forces FAmax <FQmax/μTmin) apply, only FQmax is to be used.
B The necessary minimum preload force FMmin is found by proceeding as follows from this figure:
B1 If the design has to use FQmax: four steps for static or dynamic shear force
B2 If the design has to use FAmax: 2 steps for dynamic and eccentric axial force
or
FQ
FQ
178810
or
1 step for dynamic and concentric or static and eccentric axial force
or
0 steps for static and concentric axial force
C The required maximum preload force FMmax is found by proceeding from force FMmin with: 2 steps for tightening the screw with a simple screw-driver which is set for a tightening torque
or
1 step for tightening with a torque wrench or precision screwdriver, which is set by means of the dynamic torque measurement or elongation of the screw
or
0 steps for tightening by angle control in the plastic range or by computerised yield point control
D Next to the number that is found, the required screw dimension in mm for the appropriate strength class for the screw is found in columns 2 to 4.
Example:A joint is subjected dynamically and eccentrically to an axial force of 9000 N (FA).The strength class was stipulated previously as strength class 10.9. The installation is carried out using a torque wrench.A 10.000 N is the next higher force in column 1 for the
force FA
B 2 additional steps because of eccentric and dynamic axial force Reading: 25,000 N (= FMmin)
C 1 additional step because of the tightening method using a torque wrench Reading: 40,000 N (= FMmax)
D The screw size M12 is now read in column 3 for strength class 10.9.
6.2 Choosing the tightening method and the mode of procedure
Tightening factor αA (taking the tightening uncertainty into account)All tightening methods are more or less accurate. This is caused by:
the large range of distribution of the friction that • actually occurs during installation (friction figures can only be estimated roughly for the calculation) differences in the manipulation with the torque wrench • (e.g. fast or slow tightening of the screw)
Depending on whether the influences referred to above can be controlled. the tightening factor αA has to be selected.
FA
FA
FA
FA
FA
FA
FA
FA
101789
A calculation is therefore made taking account of the tightening and setting method, as well as the coefficients of friction classes in accordance with the following table.
Reference values for the tightening factor αA
Tightening factor αA
Distribution Tightening method
Setting method Notes
1.05 to 1.2 ±2% to ±10% Elongation-controlled tightening with ultrasound
Sound transmission time
Calibration values required• With l• K/d<2 progressive fault increase to be notedSmaller fault with direct mechanical • coupling, greater fault with indirect coupling
1.1 to 1.5 ±5% to ±20% Mechanical length measuring
Setting by means of elongation measure-ment
The exact determination of the screw's • axial elastic flexibility is necessary. The distribution depends essentially on the accuracy of the measuring method.With l• K/d<2 progressive fault increase to be noted
1.2 to 1.4 ±9% to ±17% Yield strength controlled tightening, power-operated or manual
Input of the relative torque – angle of rotation coefficients
The preload force distribution is determined basically through the distribution of the yield point in the installed screw batch. The screws are dimensioned here for FMmin. A construc-tion of the screws for FMmax with the tightening factor αA is therefore not applicable for these tightening methods.
1.2 to 1.4 ±9% to ±17% Rotation angle controlled tightening, power-operated or manual
Experimental determi-nation of preliminary torque and angle of rotation (stages)
1.2 to 1.6 ±9% to ±23% Hydraulic tightening Setting by means of length or pressure measuring
Lower values for long screws (l• K/d≥5)Higher values for short screws (l• K/d≤2)
1.4 to 1.6 ±17% to ±23% Torque controlled tightening with torque wrench, torque signalling wrench or mechanical screw driver with dynamic torque measuring
Experimental deter-mination of target tor-ques at the original screw part, e.g. by means of elongation measurements of the screw
Lower values: large number of setting or control tests neces-sary (e.g. 20). Low distribution of the given torque (e.g. ±5%) necessary.
Lower values for:low angle of rotation, i.e. relatively stiff con-nectionsrelatively low hard-ness of the counter-surfaceCounter-surfaces that do not tend to “seize”, e.g. phosphated or with sufficient lubrication.Higher values for:large angle of rotation, i.e. relatively resilient connections and fine threadsVery hard counter-surfaces in combina-tion with rough surface.
1.6 to 2.0(coefficient of friction class B)
±23% to ±33% Torque controlled tightening with torque wrench, torque signalling wrench or mechanical screw driver with dynamic torque measuring
Determining the target torques by estimating the coefficient of friction (surface- and lubrica-tion ratios)
Lower values for: measuring torque wrench on even tightening and for precision torque wrenchesHigher values for:signalling or collaps-ing torque wrench
1.7 to 2.5(coefficient of friction class A)
±26% to ±43%
2.5 to 4 ±43% to ±60% Tightening with impact or impulse screw driver
Setting the screws by means of the retight-ening torque, which comprises the target tightening torque (for the estimated coef-ficient of friction) and a supplement
Lower values for:large number of setting tests (retightening • torque)on the horizontal branch of the screw • driver characteristicsimpulse transmission without play•
Tab. 2
179010
A different coefficient of friction “μ” has to be selected, depending on the surface and lubrication condition of the screws or nut coat. With the great number of surface and lubrication conditions it is often difficult to ascertain the correct coefficient of friction. If the coefficient of friction is not known exactly, the lowest probable coefficient of friction is to be reckoned with so that the screw is not overloaded.
Coefficient of friction class B should be aimed for, so that the highest possible preload force with simultaneous low distribution can be applied. (The table applies to room temperature.)
6.4 Assembly preload forces FMTab and tightening torques MA with 90% utilisation of the screw yield strength Rel or 0.2% offset yield point Rp0.2 for set screws with metric standard thread in accordance with DIN ISO 262; head sizes of hexagon head screws in accordance with DIN EN ISO 4014 to 4018, screws with external hex-alobular drive in accordance with DIN 34800 or socket cap screws in accordance with DIN EN ISO 4762 and “medium” bore in accordance with DIN EN 20 273 (in accordance with VDI 2230)
6.3 Allocation of friction coefficients with reference values to different materials/surfaces and lubrication conditions in screw assemblies (in accordance with VDI 2230)
Coefficient of friction class
Range for µG and µK Selection of typical examples forMaterial/surface Lubricants
A 0.04 to 0.10 Bright metalBlack annealedPhosphateGalv. coatings such as Zn, Zn/Fe, Zn/Ni, zinc flake coatings
Solid lubricants such as MoS2, graphite, PTFE, PA, PE, PI in solid film lubricants, as top coats or in pastes; liquefied wax; wax dispersions
B 0.08 to 0.16 Bright metalBlack annealedPhosphateGalv. coatings such as Zn, Zn/Fe, Zn/Ni, zinc flake coatings, Al and Mg alloys
Solid lubricants such as MoS2, graphite, PTFE, PA, PE, PI in solid film lubricants, as top coats or in pastes; liquefied wax; wax dispersions; greases, oils, delivery condition
Hot dip galvanised MoS2; graphite; wax dispersionsOrganic coating With integrated solid lubricant or wax
dispersionAustenitic steel Solid lubricants, waxes, pastes
C 0.14 to 0.24 Austenitic steel Wax dispersions, pastesBright metal, Phosphate Delivery condition (lightly oiled)Galv. coatings such as Zn, Zn/Fe, Zn/Ni, zinc flake coatings, adhesive
None
D 0.20 to 0.35 Austenitic steel OilGalv. coatings such as Zn, Zn/Fe, hot-dip galvanised
None
E ≥ 0.30 Galv. coatings such as Zn/Fe, Zn/Ni, austenitic steel, Al, Mg alloys
None
Tab. 3
101791
Standard thread
Size Strength class
Assembly preload forces FMTab in kN for μG =
Tightening torquesMA in Nm for μK = μG =
0.08 0.10 0.12 0.14 0.16 0.20 0.24 0.08 0.10 0.12 0.14 0.16 0.20 0.24
M4 8.810.912.9
4.66.88.0
4.56.77.8
4.46.57.6
4.36.37.4
4.26.17.1
3.95.76.7
3.75.46.3
2.33.33.9
2.63.94.5
3.04.65.1
3.34.85.6
3.65.36.2
4.16.07.0
4.56.67.8
M5 8.810.912.9
7.611.113.0
7.410.812.7
7.210.612.4
7.010.312.0
6.810.011.7
6.49.411.0
6.08.810.3
4.46.57.6
5.27.68.9
5.98.610.0
6.59.511.2
7.110.412.2
8.111.914.0
9.013.215.5
M6 8.810.912.9
10.715.718.4
10.415.317.9
10.214.917.5
9.914.517.0
9.614.116.5
9.013.215.5
8.412.414.5
7.711.313.2
9.013.215.4
10.114.917.4
11.316.519.3
12.318.021.1
14.120.724.2
15.622.926.8
M7 8.810.912.9
15.522.726.6
15.122.526.0
14.821.725.4
14.421.124.7
14.020.524.0
13.119.322.6
12.318.121.2
12.618.521.6
14.821.725.4
16.824.728.9
18.727.532.2
20.530.135.2
23.634.740.6
26.238.545.1
M8 8.810.912.9
19.528.733.6
19.128.032.8
18.627.332.0
18.126.631.1
17.625.830.2
16.524.328.4
15.522.726.6
18.527.231.8
21.631.837.2
24.636.142.2
27.340.146.9
29.843.851.2
34.350.358.9
38.055.865.3
M10 8.810.912.9
31.045.653.3
30.344.552.1
29.643.450.8
28.842.249.4
27.941.048.0
26.338.645.2
24.736.242.4
365362
436373
487183
547993
5987101
68100116
75110129
M12 8.810.912.9
45.266.377.6
44.164.875.9
43.063.274.0
41.961.572.0
40.759.870.0
38.356.365.8
35.952.861.8
6392108
73108126
84123144
93137160
102149175
117172201
130191223
M14 8.810.912.9
62.091.0106.5
60.688.9104.1
59.186.7101.5
57.584.498.8
55.982.196.0
52.677.290.4
49.372.584.8
100146171
117172201
133195229
148218255
162238279
187274321
207304356
M16 8.810.912.9
84.7124.4145.5
82.9121.7142.4
80.9118.8139.0
78.8115.7135.4
76.6112.6131.7
72.2106.1124.1
67.899.6116.6
153224262
180264309
206302354
230338395
252370433
291428501
325477558
M18 8.810.912.9
107152178
104149174
102145170
99141165
96137160
91129151
85121142
220314367
259369432
295421492
329469549
360513601
415592692
462657769
M20 8.810.912.9
136194227
134190223
130186217
127181212
123176206
116166194
109156182
308438513
363517605
415592692
464661773
509725848
588838980
6559331,092
M22 8.810.912.9
170242283
166237277
162231271
158225264
154219257
145207242
137194228
417595696
495704824
567807945
6349041,057
6979931,162
8081,1511,347
9011,2841,502
M24 8.810.912.9
196280327
192274320
188267313
183260305
178253296
168239279
157224262
529754882
6258901,041
7141,0171,190
7981,1361,329
8751,2461,458
1,0111,4401,685
1,1261,6041,877
M27 8.810.912.9
257367429
252359420
246351410
240342400
234333389
220314367
207295345
7721,1001,287
9151,3041,526
1,0501,4961,750
1,1761,6741,959
1,2921,8402,153
1,4982,1342,497
1,6722,3812,787
M30 8.810.912.9
313446522
307437511
300427499
292416487
284405474
268382447
252359420
1,0531,5001,755
1,2461,7752,077
1,4282,0332,380
1,5972,2742,662
1,7542,4982,923
2,9312,8933,386
2,2653,2263,775
M33 8.810.912.9
389554649
381543635
373531621
363517605
354504589
334475556
314447523
1,4152,0152,358
1,6792,3222,799
1,9282,7473,214
2,1613,0783,601
2,3773,3853,961
2,7593,9304,598
3,0814,3885,135
M36 8.810.912.9
458652763
448638747
438623729
427608711
415591692
392558653
368524614
1,8252,6003,042
2,1643,0823,607
2,4823,5354,136
2,7783,9574,631
3,0544,3495,089
3,5415,0435,902
3,9515,6276,585
M39 8.810.912.9
548781914
537765895
525748875
512729853
498710831
470670784
443630738
2,3483,3453,914
2,7913,9754,652
3,2084,5695,346
3,5975,1235,994
3,9585,6376,596
4,5986,5497,664
5,1377,3178,562
Tab. 5
179210
Assembly preload forces FMTab and tightening torques MA with 90% utilisation of the screw yield strength Rel or 0.2% offset yield point Rp0.2 for set screws with metric fine thread in accordance with DIN ISO 262; head sizes of hexagon head screws in accordance with DIN EN ISO 4014 to 4018, screws with external hexalobular drive in accordance with DIN 34800 or socket cap screws in accordance with DIN EN ISO 4762 and “medium” bore in accordance with DIN EN 20 273 (in accordance with VDI 2230)
Fine threadSize Strength
classAssembly preload forces FMTab in kN for μG =
Tightening torquesMA in Nm for μK = μG =
0.08 0.10 0.12 0.14 0.16 0.20 0.24 0.08 0.10 0.12 0.14 0.16 0.20 0.24
M8x 1
8.810.912.9
21.231.136.4
20.730.435.6
20.229.734.7
19.728.933.9
19.228.132.9
18.126.531.0
17.024.929.1
19.328.433.2
22.833.539.2
26.138.344.9
29.242.850.1
32.047.055.0
37.054.363.6
41.260.570.8
M9x 1
8.810.912.9
27.740.747.7
27.239.946.7
26.539.045.6
25.938.044.4
25.237.043.3
23.734.940.8
22.332.838.4
28.041.148.1
33.248.857.0
38.155.965.4
42.662.673.3
46.968.880.6
54.479.893.4
60.789.1104.3
M10x 1
8.810.912.9
35.251.760.4
34.550.659.2
33.749.557.9
32.948.356.5
32.047.055.0
30.244.451.9
28.441.748.8
395767
466880
537891
6088103
6697113
76112131
85125147
M10x 1,25
8.810.912.9
33.148.656.8
32.447.555.6
31.646.454.3
30.845.252.9
29.944.051.4
28.241.448.5
26.538.945.5
385565
446576
517587
578398
6292107
72106124
80118138
M12x 1,25
8.810.912.9
50.173.686.2
49.172.184.4
48.070.582.5
46.868.780.4
45.666.978.3
43.063.273.9
40.459.469.5
6697114
79116135
90133155
101149174
111164192
129190222
145212249
M12x 1,5
8.810.912.9
47.670.081.9
46.668.580.1
45.566.878.2
44.365.176.2
43.163.374.1
40.659.769.8
38.256.065.6
6495111
76112131
87128150
97143167
107157183
123181212
137202236
M14x 1,5
8.810.912.9
67.899.5116.5
66.497.5114.1
64.895.2111.4
63.292.9108.7
61.590.4105.8
58.185.399.8
45.680.293.9
104153179
124182213
142209244
159234274
175257301
203299349
227333390
M16x 1,5
8.810.912.9
91.4134.2157.1
89.6131.6154.0
87.6128.7150.6
85.5125.5146.9
83.2122.3143.1
78.6155.5135.1
74.0108.7127.2
159233273
189278325
218320374
244359420
269396463
314461539
351515603
M18x 1,5
8.810.912.9
122174204
120171200
117167196
115163191
112159186
105150176
99141166
237337394
283403472
327465544
368523613
406578676
473674789
530755884
M18x 2
8.810.912.9
114163191
112160187
109156182
107152178
104148173
98139163
92131153
229326381
271386452
311443519
348496581
383545638
444632740
495706826
M20x 1,5
8.810.912.9
154219257
151215252
148211246
144206241
141200234
133190222
125179209
327466545
392558653
454646756
511728852
565804941
6609401,100
7411,0551,234
M22x 1,5
8.810.912.9
189269315
186264309
182259303
178253296
173247289
164233273
154220257
440627734
529754882
6138731,022
6929851,153
7651,0901,275
8961,2761,493
1,0061,4331,677
M24x 1,5
8.810.912.9
228325380
224319373
219312366
214305357
209298347
198282330
187266311
570811949
6869771,143
7961,1331,326
8991,2801,498
9951,4171,658
1,1661,6611,943
1,3111,8672,185
M24x 2
8.810.912.9
217310362
213304355
209297348
204290339
198282331
187267312
177251294
557793928
6669491,110
7691,0951,282
8651,2321,442
9551,3601,591
1,1141,5861,856
1,2481,7772,080
M27x 1,5
8.810.912.9
293418489
288410480
282402470
276393460
269383448
255363425
240342401
8221,1711,370
9921,4131,654
1,1531,6431,922
1,3041,8582,174
1,4452,0592,409
1,6972,4172,828
1,9102,7203,183
101793
6.5 Tightening torque and preload force ofSafety screws with nuts• Flange screws with nuts•
With 90% utilisation of the screws’ yield strength Rel or 0.2% offset yield point Rp0.2 (according to manufacturer’s data)
Size Strength class
Assembly preload forces FMTab in kN for μG =
Tightening torquesMA in Nm for μK = μG =
0.08 0.10 0.12 0.14 0.16 0.20 0.24 0.08 0.10 0.12 0.14 0.16 0.20 0.24
M27x 2
8.810.912.9
281400468
276393460
270384450
264375439
257366428
243346405
229326382
8061,1491,344
9671,3781,612
1,1191,5941,866
1,2621,7972,103
1,3941,9862,324
1,6302,3222,717
1,8292,6053,049
M30x 2
8.810.912.9
353503588
347494578
339483565
331472552
323460539
306436510
288411481
1,1161,5901,861
1,3431,9122,238
1,5562,2162,594
1,7562,5022,927
1,9432,7673,238
2,2763,2413,793
2,5573,6414,261
M33x 2
8.810.912.9
433617722
425606709
416593694
407580678
397565662
376535626
354505591
1,4892,1202,481
1,7942,5552,989
2,0822,9653,470
2,3523,3503,921
2,6053,7104,341
3,0544,3505,090
3,4354,8925,725
M36x 2
8.810.912.9
521742869
512729853
502714836
490698817
478681797
453645755
427609712
1,9432,7673,238
2,3453,3403,908
2,7253,8824,542
3,0824,3905,137
3,4154,8645,692
4,0105,7116,683
4,5136,4287,522
M39x 2
8.810.912.9
6188801,030
6078641,011
595847991
581828969
567808945
537765896
507722845
2,4833,5374,139
3,0024,2765,003
3,4934,9745,821
3,9535,6316,589
4,3836,2437,306
5,1517,3368,585
5,8018,2639,669
Tab. 6
Counter material
Preload forces FVmax (N) Tightening torque MA (Nm)
M5 M6 M8 M10 M12 M14 M16 M5 M6 M8 M10 M12 M14 M16
Locking screwsstrength class 100 and nuts strength class 10
SteelRm < 800 MPa
9,000 12,600 23,200 37,000 54,000 74,000 102,000 11 19 42 85 130 230 330
SteelRm = 800– 1,100 MPa
9,000 12,600 23,200 37,000 54,000 74,000 102,000 10 18 37 80 120 215 310
Gray cast iron
9,000 12,600 23,200 37,000 54,000 74,000 102,000 9 16 35 75 115 200 300
Reference values
179410
6.6 Reference values for tightening torques for austenite screws in accordance with DIN EN ISO 3506
The following table shows the tightening torque required for an individual case in dependence on the nominal diameter, the coefficient of friction and the strength class (SC) as a reference value.
Coefficient of friction µges 0.10
Preload forces FVmax. [KN]
Tightening torque MA [Nm]
FK 50 FK 70 FK 80 FK 50 FK 70 FK80
M3 0.90 1.00 1.20 0.85 1.00 1.30
M4 1.08 2.97 3.96 0.80 1.70 2.30
M5 2.26 4.85 6.47 1.60 3.40 4.60
M6 3.2 6.85 9.13 2.80 5.90 8.00
M8 5.86 12.6 16.7 6.80 14.5 19.3
M10 9.32 20.0 26.6 13.7 30.0 39.4
M12 13.6 29.1 38.8 23.6 50.0 67.0
M14 18.7 40.0 53.3 37.1 79.0 106.0
M16 25.7 55.0 73.3 56.0 121.0 161.0
M18 32.2 69.0 92.0 81.0 174.0 232.0
M20 41.3 88.6 118.1 114.0 224.0 325.0
M22 50.0 107.0 143.0 148.0 318.0 424.0
M24 58.0 142.0 165.0 187.0 400.0 534.0
M27 75.0 275.0
M30 91.0 374.0
M33 114.0 506.0
M36 135.0 651.0
M39 162.0 842.0
Coefficient of friction µges 0.20
Preload forces FVmax. [KN]
Tightening torque MA [Nm]
FK 50 FK 70 FK 80 FK 50 FK 70 FK 80
M3 0.60 0.65 0.95 1.00 1.10 1.60
M4 1.12 2.40 3.20 1.30 2.60 3.50
M5 1.83 3.93 5.24 2.40 5.10 6.90
M6 2.59 5.54 7.39 4.10 8.80 11.8
M8 4.75 10.2 13.6 10.1 21.4 28.7
M10 7.58 16.2 21.7 20.3 44.0 58.0
M12 11.1 23.7 31.6 34.8 74.0 100.0
M14 15.2 32.6 43.4 56.0 119.0 159.0
M16 20.9 44.9 59.8 86.0 183.0 245.0
M18 26.2 56.2 74.9 122.0 260.0 346.0
M20 33.8 72.4 96.5 173.0 370.0 494.0
M22 41.0 88.0 118.0 227.0 488.0 650.0
M24 47.0 101.0 135.0 284.0 608.0 810.0
M27 61.0 421.0
M30 75.0 571.0
M33 94.0 779.0
M36 110.0 998.0
M39 133.0 1.300
Coefficient of friction µges 0.30
Preload forces FVmax. [KN]
Tightening torque MA [Nm]
FK 50 FK 70 FK 80 FK 50 FK 70 FK80
M3 0.40 0.45 0.70 1.25 1.35 1.85
M4 0.90 1.94 2.59 1.50 3.00 4.10
M5 1.49 3.19 4.25 2.80 6.10 8.00
M6 2.09 4.49 5.98 4.80 10.4 13.9
M8 3.85 8.85 11.0 11.9 25.5 33.9
M10 6.14 13.1 17.5 24.0 51.0 69.0
M12 9.00 19.2 25.6 41.0 88.0 117.0
M14 12.3 26.4 35.2 66.0 141.0 188.0
M16 17.0 36.4 48.6 102.0 218.0 291.0
M18 21.1 45.5 60.7 144.0 308.0 411.0
M20 27.4 58.7 78.3 205.0 439.0 586.0
M22 34.0 72.0 96.0 272.0 582.0 776.0
M24 39.0 83.0 110.0 338.0 724.0 966.0
M27 50.0 503.0
M30 61.0 680.0
M33 76.0 929.0
M36 89.0 1.189
M39 108.0 1.553
Tab. 8
101795
6.7 How to use the tables for preload forces and tightening torques!
The procedure is as follows:
A) Determining the total coefficient of friction μges.:
Different coefficients of friction “μ” have to be reckoned with, depending on the surface or lubrication condition of the screws or nuts. Table 3 in chapter 6 is used to make the selection.
Example:Selecting the screw and nut with surface condition zinc galvanised transparent passivation, without lubricant:µges = 0.14 (Note: the lowest probable coefficient of friction is to be reckoned with for the dimensioning of the screw so that it is not overloaded)
B) Tightening torque MA max.The maximum tightening torque is found with 90% utilisa-tion of the 0.2% offset yield point (Rp0.2) or of the yield point (Rel).
Example:Hexagon head screw DIN 933, M12 x 50, strength class 8.8, galvanised, blue passivation:In Table 5 in chapter 6 look in the column for μG = 0.14 for the line for M12 with strength class 8.8.Now read off the desired valueMA max = 93 Nmfrom the section “Tightening torque MA [Nm]”.
C) Tightening factor αA (taking the tightening uncertainty into account)
All tightening methods are more or less accurate. This is caused by:
The large range of distribution of the friction that • actually occurs during installation (if friction figures can only be estimated roughly for the calculation)Differences in the manipulation with the torque wrench • (e.g. fast or slow tightening of the screw) The distribution of the torque wrench itself.•
Depending on how the above-mentioned influences are controlled, the tightening factor αA must be selected.
Example:If a commercially available torque wrench with an electronic display is used, a tightening factorαA = 1.4–1.6 must be reckoned with.The selection is:αA = 1. 4 (see Table 2 in chapter 6 “Reference values for the tightening factor ...”)
D) Preload force FVmin
Example:In Table 5 in chapter 6 in column μG = 0.14, line M12 and strength class 8.8 read off the value for the maximum preload force FVmax = 41.9 KN in the area “Assembly preload forces”.
The minimum preload force FVmin is obtained by dividing FVmax by the tightening factor αA.
Preload force FVmin = 41.9 KN
1.4
FVmin = 29.92 KN
E) Control of the resultsYou should ask yourself the following questions!
Is the residual clamping power sufficient? • Is the minimum probable preload force F• Vmin sufficient for the maximum forces that occur in practice?
179610
6.8 Pairing different element/contact corrosion The following rule applies for preventing contact corrosion:In each case fasteners must have at least the same corrosion resistance as the parts that are to be connected. If fasteners of equal value cannot be selected, they must be of higher value than the parts to be connected.
Pairing different fasteners/component materials with regard to contact corrosion
Component material/surface*
Fastener material/surface Stai
nles
s ste
el A
2/A4
Alum
iniu
m
Copp
er
Bras
s
Stee
l, ga
lvani
sed,
bla
ck p
ass.
Stee
l, ga
lvani
sed,
yel
low
chr
omat
ed
Stee
l, ga
lvani
sed,
blu
e pa
ss.
Stee
l, br
ight
Stainless steel A2/A4 +++ +++ ++ ++ ++ ++ ++ ++
Aluminium ++ +++ ++ ++ + + + +
Copper + + +++ ++ + + + +
Brass + + ++ +++ + + + +
Steel, galvanised, black passivated – – – – +++ ++ ++ +
Steel, galvanised, yellow chromated –– –– –– –– + +++ ++ +
Steel, galvanised, blue passivated –– –– –– –– + + +++ +
Steel, bright ––– ––– ––– ––– –– –– –– +++
+++ Highly recommended pairing++ Recommended pairing+ Moderately recommended pairing– Less recommended pairing–– Not recommended pairing––– Pairing not recommended under any circumstances* This assumption applies with a surface ratio (component ratio of fastener to the
part to be connected) between 1:10 and 1:40.
Tab. 9
Nominal diameter [mm] 1 1.5 2 2.5 3 3.5 4 4.5 5 6 8 10 12 13 14 16 18 20
Shearing force min. [kN] Single-shear 0.35 0.79 1.41 2.19 3.16 4.53 5.62 7.68 8.77 13 21.3 35 52 57.5 72.3 85.5 111.2 140.3
Two-shear 0.7 1.58 2.82 4.38 6.32 9.06 11.2 15.4 17.5 26 42.7 70.1 104.1 115.1 144.1 171 222.5 280.6
Tab. 10
6.9 Static shearing forces for slotted spring pin connectionsSlotted spring pins, heavy duty in accordance with ISO 8752 (DIN 1481)
Up to 10 mm nominal diameterUp to 8 mm nominal diameter
Fig. AU Fig. AV
Material:Spring steel hardenedfrom 420 to 560 HV
101797
Nominal diameter [mm] 0.8 1 1.2 1.5 2 2.5 3 3.5 4 5 6 8 10 12 14 16
Shearing force min. [kN] Single-shear 0.21 0.3 0.45 0.73 1.29 1.94 2.76 3.77 4.93 7.64 11.05 19.6 31.12 44.85 61.62 76.02
Two-shear 0.40 0.6 0.90 1.46 2.58 3.88 5.52 7.54 9.86 15.28 22.1 39.2 62.24 89.7 123.2 152
Tab. 11
Spring-type straight pins, standard design in accordance with ISO 8750 (DIN 7343)
Fig. AW
Material:Spring steel hardenedfrom 420 to 520 HV
Nominal diameter [mm] 1.5 2 2.5 3 4 5 6
Shearing force min. [kN] Single-shear 0.91 1.57 2.37 3.43 6.14 9.46 13.5
Two-shear 1.82 3.14 4.74 6.86 12.2 18.9 27
Tab. 12
Spring-type straight pins, coiled, heavy duty in accordance with ISO 8748 (DIN 7344)
Fig. AX
Material:Spring steel hardenedfrom 420 to 520 HV
Nominal diameter [mm] 2 2.5 3 3.5 4 4.5 5 6 7 8 10 11 12 13 14 16 18 20
Shearing force min. [kN] Single-shear 0.75 1.2 1.75 2.3 4 4.4 5.2 9 10.5 12 20 22 24 33 42 49 63 79
Two-shear 1.5 2.4 3.5 4.6 8 8.8 10.4 18 21 24 40 44 48 66 84 98 126 158
Tab. 13
Spring-type straight pins, slotted, light duty in accordance with ISO 13337 (DIN 7346)
Up to 10 mm nominal diameterUp 8 mm nominal diameter
Fig. AY Fig. AZ
Material:Spring steel hardenedfrom 420 to 560 HV
179810
6.10 Design recommendations for internal drives for screws
Technical progress and financial considerations are leading worldwide to an almost complete replacement of straight slot screws by internal drives.
AW drive
Fig. AR
AW drive systemAdvantages with regard to previous drive systems:
Improved force transmission by means of the conical • multipoint head.Longer service life through optimal fit.• Optimum centring through the conical course of the bit.• Greatest possible bearing surface of the bit in the • screw drive → comeout.Comeout = zero. The even force distribution prevents • damage to the surface protective layer and thus guar-antees greater corrosion resistance.
Hexagonal socket
Fig. AS
Good force transmission through several points of ap-plication of force. Hexagonal socket-screws have smaller widths across flats than hexagon head screws, which also means more economic designs because of smaller dimensions.
Cross recess Z (pozi drive) in accordance with ISO 4757
Fig. AT
Previous drive systems AW drive
single-shear
F
F
two-shear
2F
F F
Fig. BA
101799
The four “tightening walls” in the cross recess, with which the screwdriver is in contact when the screw is being screwed in, are vertical. The remaining walls and ribs are slanted. This can improve ease of assembly if the cross recesses are made optimally. Pozi drive screwdrivers have rectangular blade ends.
Cross recess H (Phillips) in accordance with ISO 4757
Fig. AU
Normal cross recess in which all walls and ribs are slanted, whereby the screwdriver has trapezoid blade ends.
6.11 AssemblyTorque methodThe required preload force is generated by the measurable torque MV. The tightening appliance that is used (e.g. a torque wrench) must have uncertainty of less than 5%.
Angular momentum methodThe connections are tightened with the help of an impulse or impact driver with an uncertainty of less than 5%. The tightening appliances are to be adjusted as far as pos-sible to the original screw assembly in a suitable manner (e.g. retightening method or length measuring method).
Retightening method: the connection is tightened first of all with the screwdriver and then retightened/checked with a precision torque wrench. Length measuring method: the resulting lengthening of the screw is checked (measuring calliper), whereby the lengthening of the screw has to be calibrated beforehand on a screw test stand.
Angle of rotation methodPrerequisite is that the parts to be joined rest largely flat on each other. The pre-tightening torque is applied with one of the two methods described above. Mark the posi-tion of the nut relative to the screw shaft and component clearly and permanently, so that the subsequently applied further tightening angle of the nut can be determined easily. The required further tightening angle must be determined by means of a method test at the respective original screwed connections (e.g. by means of screw lengthening).
Fig. W
Top Related